RFC 7730

Resource Public Key Infrastructure (RPKI) Trust Anchor Locator

Internet Engineering Task Force (IETF) G. Huston
Request for Comments: 7730 APNIC
Obsoletes: 6490 S. Weiler
Category: Standards Track Parsons
ISSN: 2070-1721 G. Michaelson
APNIC
S. Kent
BBN
January 2016 Resource Public Key Infrastructure (RPKI) Trust Anchor Locator
Abstract
This document defines a Trust Anchor Locator (TAL) for the Resource
Public Key Infrastructure (RPKI). This document obsoletes RFC 6490
by adding support for multiple URIs in a TAL.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7730.
Copyright Notice
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extensions that represent Internet Number Resources (INRs) [RFC3779].
The set of INRs associated with an entity acting as a trust anchor is
likely to change over time. Thus, if one were to use the common PKI
convention of distributing a trust anchor to RPs in a secure fashion,
then this procedure would need to be repeated whenever the INR set
for the entity acting as a trust anchor changed. By distributing the
TAL (in a secure fashion), instead of distributing the trust anchor,
this problem is avoided, i.e., the TAL is constant so long as the
trust anchor's public key and its location do not change.
The TAL is analogous to the TrustAnchorInfo data structure specified
in [RFC5914], which is on the Standards Track. That specification
could be used to represent the TAL, if one defined an rsync URI
extension for that data structure. However, the TAL format was
adopted by RPKI implementors prior to the PKIX trust anchor work, and
the RPKI implementer community has elected to utilize the TAL format,
rather than define the requisite extension. The community also
prefers the simplicity of the ASCII encoding of the TAL, versus the
binary (ASN.1) encoding for TrustAnchorInfo.
The TAL is an ordered sequence of:
1) a URI section,
2) a <CRLF> or <LF> line break,
3) a subjectPublicKeyInfo [RFC5280] in DER format [X.509],
encoded in Base64 (see Section 4 of [RFC4648]). To avoid long
lines, <CRLF> or <LF> line breaks MAY be inserted into the
Base64-encoded string.
where the URI section is comprised of one of more of the ordered
sequence of:
1.1) an rsync URI [RFC5781],
1.2) a <CRLF> or <LF> line break.
2.2. TAL and Trust Anchor Certificate Considerations
Each rsync URI in the TAL MUST reference a single object. It MUST
NOT reference a directory or any other form of collection of objects.
The referenced object MUST be a self-signed CA certificate that
conforms to the RPKI certificate profile [RFC6487]. This certificate
is the trust anchor in certification path discovery [RFC4158] and
validation [RFC5280] [RFC3779].

The validity interval of this trust anchor SHOULD reflect the
anticipated period of stability of the particular set of INRs that
are associated with the putative trust anchor.
The INR extension(s) of this trust anchor MUST contain a non-empty
set of number resources. It MUST NOT use the "inherit" form of the
INR extension(s). The INR set described in this certificate is the
set of number resources for which the issuing entity is offering
itself as a putative trust anchor in the RPKI [RFC6480].
The public key used to verify the trust anchor MUST be the same as
the subjectPublicKeyInfo in the CA certificate and in the TAL.
The trust anchor MUST contain a stable key. This key MUST NOT change
when the certificate is reissued due to changes in the INR
extension(s), when the certificate is renewed prior to expiration, or
for any reason other than a key change.
Because the public key in the TAL and the trust anchor MUST be
stable, this motivates operation of that CA in an offline mode.
Thus, the entity that issues the trust anchor SHOULD issue a
subordinate CA certificate that contains the same INRs (via the use
of the "inherit" option in the INR extensions of the subordinate
certificate). This allows the entity that issues the trust anchor to
keep the corresponding private key of this certificate offline, while
issuing all relevant child certificates under the immediate
subordinate CA. This measure also allows the Certificate Revocation
List (CRL) issued by that entity to be used to revoke the subordinate
CA certificate in the event of suspected key compromise of this
online operational key pair that is potentially more vulnerable.
The trust anchor MUST be published at a stable URI. When the trust
anchor is reissued for any reason, the replacement CA certificate
MUST be accessible using the same URI.
Because the trust anchor is a self-signed certificate, there is no
corresponding CRL that can be used to revoke it, nor is there a
manifest [RFC6486] that lists this certificate.
If an entity wishes to withdraw a self-signed CA certificate as a
putative trust anchor, for any reason, including key rollover, the
entity MUST remove the object from the location referenced in the
TAL.
Where the TAL contains two or more rsync URIs, then the same self-
signed CA certificate MUST be found at each referenced location. In
order to increase operational resilience, it is RECOMMENDED that the
domain name parts of each of these URIs resolve to distinct IP

addresses that are used by a diverse set of repository publication
points, and these IP addresses be included in distinct Route Origin
Authorizations (ROAs) objects signed by different CAs.
2.3. Example
rsync://rpki.example.org/rpki/hedgehog/root.cer
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAovWQL2lh6knDx
GUG5hbtCXvvh4AOzjhDkSHlj22gn/1oiM9IeDATIwP44vhQ6L/xvuk7W6
Kfa5ygmqQ+xOZOwTWPcrUbqaQyPNxokuivzyvqVZVDecOEqs78q58mSp9
nbtxmLRW7B67SJCBSzfa5XpVyXYEgYAjkk3fpmefU+AcxtxvvHB5OVPIa
BfPcs80ICMgHQX+fphvute9XLxjfJKJWkhZqZ0v7pZm2uhkcPx1PMGcrG
ee0WSDC3fr3erLueagpiLsFjwwpX6F+Ms8vqz45H+DKmYKvPSstZjCCq9
aJ0qANT9OtnfSDOS+aLRPjZryCNyvvBHxZXqj5YCGKtwIDAQAB
3. Relying Party Use
In order to use the TAL to retrieve and validate a (putative) trust
anchor, an RP SHOULD:
1. Retrieve the object referenced by (one of) the URI(s) contained
in the TAL.
2. Confirm that the retrieved object is a current, self-signed RPKI
CA certificate that conforms to the profile as specified in
[RFC6487].
3. Confirm that the public key in the TAL matches the public key in
the retrieved object.
4. Perform other checks, as deemed appropriate (locally), to ensure
that the RP is willing to accept the entity publishing this self-
signed CA certificate to be a trust anchor. These tests apply to
the validity of attestations made in the context of the RPKI
relating to all resources described in the INR extension of this
certificate.
An RP SHOULD perform these functions for each instance of TAL that it
is holding for this purpose every time the RP performs a
resynchronization across the local repository cache. In any case, an
RP also SHOULD perform these functions prior to the expiration of the
locally cached copy of the retrieved trust anchor referenced by the
TAL.

In the case where a TAL contains multiple URIs, an RP MAY use a
locally defined preference rule to select the URI to retrieve the
self-signed RPKI CA certificate that is to be used as a trust anchor.
Some examples are:
o Using the order provided in the TAL
o Selecting the URI randomly from the available list
o Creating a prioritized list of URIs based on RP-specific
parameters, such as connection establishment delay
If the connection to the preferred URI fails, or the retrieved CA
certificate public key does not match the TAL public key, the RP
SHOULD retrieve the CA certificate from the next URI, according to
the local preference ranking of URIs.
4. Security Considerations
Compromise of a trust anchor private key permits unauthorized parties
to masquerade as a trust anchor, with potentially severe
consequences. Reliance on an inappropriate or incorrect trust anchor
has similar potentially severe consequences.
This TAL does not directly provide a list of resources covered by the
referenced self-signed CA certificate. Instead, the RP is referred
to the trust anchor itself and the INR extension(s) within this
certificate. This provides necessary operational flexibility, but it
also allows the certificate issuer to claim to be authoritative for
any resource. Relying parties should either have great confidence in
the issuers of such certificates that they are configuring as trust
anchors, or they should issue their own self-signed certificate as a
trust anchor and, in doing so, impose constraints on the subordinate
certificates.
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779,
DOI 10.17487/RFC3779, June 2004,
<http://www.rfc-editor.org/info/rfc3779>.